Javascript – Large post request truncated when submitted by browser, okay in Postman

I am using a javascript fetch post call to submit form data that is over 1mb.
A backend perl/cgi script writes the data to a file.

When submitted via chrome/IE the data is truncated at 1mb and doesn’t save.
Postman calls using the raw data from the broswer successfully runs and saves.

The content-length shows the correct length for both postman and browser

When I return the input data to the response so I can see what is happening from postman:

{6 mbs of data}
------WebKitFormBoundarylYiQSbqkKI68Lzxk--OK6887007

When I return the input data to the response so I can see what is happening from the browser:

{1 mb of data}
OK6887007

I have tested with data just under 1mb and it uploads/saves fine in the browser.

I really cannot be 100% sure it is the fetch truncating it,
the broswer truncating it, or the perl script cutting it off before the finished transmission.

In the network tab I can see the entire 6mb request, but cannot tell if it actually gets there. From reading, there doesn’t appear to be any limit I am supposed to be hitting.

My javascript function:

async function serverSave(data,access) {
    // Data to send in the POST request
    const formData = new FormData();
    formData.append('access', access);
    formData.append('data', data);
  
    // URL endpoint to send the POST request to
    const url = 'http://www.mywebsite.com/cgi-bin/upload.pl';
    // Options for the fetch request
    const options = { method: 'POST', body: formData };
    
    // Send the POST request using fetch
    try{
        response = fetch(url, options);
        const result = await response.text();
        console.log('Response:', result);
    }
    catch {
        // Handle errors
        console.error('There was a problem saving to the server', error);
    };  
}

My perl script:

#!/usr/bin/perl
use strict;
use warnings;

my $buffer = '';
if($ENV{CONTENT_LENGTH}) { read(STDIN,$buffer,$ENV{CONTENT_LENGTH}); }

my ($data_value) = $buffer =~ /name="data"s*?ns*?n(.+?)n-{5}/s;
my ($access_value) = $buffer =~ /name="access"s*?ns*?n(.+?)n-{5}/s;

$access_value =~ s/[rn]+$//;
$data_value =~ s/[rn]+$//;

my $fn = "../zfiles/".$access_value.".json";

open(FH, '>', $fn) or die $!;
print FH $data_value;
close(FH);

print "Content-type: text/plainnn";
print $buffer;
print "OK";
print $ENV{CONTENT_LENGTH};